Such vibration constitutes a major problem that needs to be combated insofar as such vibration leads to:                alternating stresses throughout the rotorcraft leading to materials fatigue phenomena and thus having a direct influence on safety;        vibration in the fuselage of the rotorcraft which can reduce the accuracy and the effectiveness of the equipments, in particular weapons mounted on the fuselage; and        vibration in the cabin, which is naturally highly disturbing for pilot and passenger comfort.        
Patent FR 2 824 395 in the name of the Applicant discloses a method of adjusting a rotor of a rotary wing aircraft. That method consists in using a reference neural network representing relationships between accelerations representative of vibration generated on at least a portion of a reference rotorcraft, and defects and adjustment parameters.
It is recalled that a neural network is conventionally made up of a set of elements operating in parallel and such that for given data inputs, the outputs of the set are characterized by the neural network. Such elements are based on biological nervous systems. As with biological neurons, the functions of a neural network are strongly determined by the connections between elements. It is thus possible to “train” a neural network so as to enable it to achieve some particular function by adjusting the values of the connections (values known as weights) between the elements. This “training” is performed or adjusted in such a manner as to ensure that each given input causes the network to deliver a specific output. On the topic of neural networks, reference can be made, for example, to an article entitled “Learning representation by back propagation errors” by D. Rumlhart, G. Hinton, and R. Williams, published in the journal Nature, 1996, Vol. 323, pp. 533 to 536.
When defects are identified on a particular rotorcraft rotor, the neural network having inlet and outlet cells is used to determine at least one adjustment parameter that is to be varied. The adjustment value α of an adjustment parameter that is to be varied is then obtained by minimizing the following expression:
      ∑    c                  ⁢          ⁢            λ      c        ⁢                  ∑        a            ⁢                          ⁢                        λ          a                ⁢                              ∑            h                    ⁢                                          ⁢                                    λ              h                        ⁢                                                                                                                      R                                              c                        ,                        a                        ,                        h                                                              ⁡                                          (                      α                      )                                                        +                                      γ                                          c                      ,                      a                      ,                      h                                                                                                  2                                          in which:                λc is a weighting coefficient depending on the stage of flight c (hovering, cruising, . . . );        λa is a weighting coefficient depending on an accelerometer a;        λh is a weighting coefficient depending on the harmonic h;        Rc,a,h is the output cell from the neural network corresponding to the harmonic h, the accelerometer a, and the stage of flight c; and        γc,a,h is an acceleration signal at the frequency h as measured by the accelerometer a for the stage of flight c.        
Nevertheless, that expression does not take account of the physiological perceptions of the occupants of the rotorcraft, i.e. the crew and the passengers. A virtual unbalance sensation remains in the cabin and this is particularly uncomfortable and even dangerous since pilot comfort is a safety's condition. This virtual unbalance is generated by a frequency beat effect caused by coupling between vibrations of harmonics B−1 and B of a rotorcraft rotor, where B is the number of blades of the rotor.